1. Field of the Invention
The present invention relates to an image forming apparatus having a fuser that thermally fuses a recording agent onto printing media, more particularly to the temperature control of the fuser.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. H7-248701 describes a method of controlling the temperature of this type of fuser by using a temperature sensor to sense the temperature of the fuser and a comparator to compare the sensed temperature with a threshold. A heater in the fuser is turned on and off according to the comparison result, using different turn-on and turn-off thresholds.
When this method is practiced, the temperature sensor generally senses the surface temperature of a fusing roller. This temperature tends to drop when the surface of the printing medium makes contact with the fusing roller. Although the temperature sensor may detect the drop and turn on the heater, it takes time for heat to reach the surface from the heater, which is located at the center of the fusing roller. The low temperature is therefore not corrected immediately, causing a so-called cold offset that can produce gloss irregularities and possible inadequate fusing.
This problem is aggravated by the relatively low fusing temperatures that are now used to reduce power consumption and increase printing speed in much image forming apparatus. To obtain adequate heating of printing media at these low fusing temperatures, the area of contact between the fusing rollers and the printing medium must be increased. The diameter of the fusing roller or the thickness of the layer of rubber on its surface has therefore been increased, so that it takes even longer for heat to reach the surface from the heater, lengthening the duration of the cold offset.
The cold offset could be mitigated by temporarily raising the turn-on threshold before each sheet of printing media entered the fuser, but this scheme would invite the opposite problem: a hot offset could occur.